Those of you with systems with subs, how well can you actually hear bass while riding? We're doing two Boston Pro 6.5 component systems on the tower, and how much sub is actually needed to hear it while riding. Or, would a setup like Pheaton's work better (2 8's and 2 6 1/2's)?

In order to play it as loud asd you will want it you are going to have to set your crossover around 120-150hz. You cannot hear any tower speaker bass 60-70 feet back. Even if you put 8" subs up there, you will not hear a ton of bass.